This invention relates to a process of purifying gases produced by a gasification of solid or liquid fossil fuels by treatment with water vapor and oxygen under superatmospheric pressure to remove catalyst-deteriorating impurities, such as mono- and polyunsaturated hydrocarbons, mercaptans, HCN, HCl, H.sub.2 S, CS.sub.2, COS, and NH.sub.3, and to desulfurize the gas, to produce an exhaust gas which is rich in H.sub.2 S, wherein the gas is cooled and scrubbed under superatmospheric pressures and at normal temperatures.
It is known that hydrogen for the synthesis of ammonia or for the catalytic hydrogenation and hydrogencarbon oxide mixtures for the catalytic synthesis of hydrocarbons (methane synthesis, Fischer-Tropsch synthesis, methanol synthesis or oxosynthesis) can be produced by gasifying various fossil fuels by a treatment with water vapor accompanied by a direct supply of heat (by a partial combustion with oxygen or air) or by an indirect supply of heat from the outside. In all cases the resulting raw gases must be purified to remove catalyst-deteriorating constituents, particularly sulfur compounds. In most cases, this is accomplished by scrubbing processes, by which the gases, which are usually produced and kept under superatmospheric pressure, are scrubbed under superatmospheric pressure with aqueous or organic scrubbing agents to remove the impurities.
In gases produced from natural gas or petroleum or fractions thereof, the catalyst-deteriorating impurities to be be removed consist mainly of H.sub.2 S, certain proportions of COS, and small amounts of HCN.
The purification of these gases is simpler than that of coal gases, which in addition to the above-mentioned constituents contain other impurities, such as NH.sub.3, CS.sub.2, mercaptans, and mono- and polyunsaturated hydrocarbons having 2 to 10 carbon atoms, i.e., acetylenes, olefins and diolefins, which tend to polymerize thermally or catalytically to form resins which deposit at various points, which results in clogging.
For the reasons stated above, the purification of coal gases has always been very expensive. This has been the case even with coke oven gases, even if they are used only as fuel gases. The purification has always included several stages for the separate removal of naphthalenes, benzene hydrocarbons, hydrogen sulfide, ammonia, and nitric oxide. Since the advent of the use of coal gases, e.g., those produced by a pressure gasification of coal, for catalytic synthesis, further stages have been added to the above-mentioned purifying processes. High concentrations of CO.sub.2 have to be removed, and the gases have been scrubbed with water under superatmospheric pressure for this purpose, and purification processes are required to remove all organic sulfur compounds. Alkalinized iron oxide compositions have been used for the latter purpose at temperatures of about 180.degree.-240.degree. C.
A new scrubbing process has been introduced (German Pat. Nos. 843,545; 935,144 and 936,714), which is based on physical absorption and in which only methanol is used as a scrubbing agent to remove all impurities from coal gases and to provide a gas which is sufficiently pure for catalytic syntheses. This process represents a great technological progress specifically in the purification of coal gases. Methanol used as a scrubbing agent for this purpose can easily be thermally regenerated without any chemical change so that it can be continually re-used. The only disadvantage of methanol is its relatively high vapor pressure. To reduce the latter and also to increase the solubility of the gases to be scrubbed, the process is carried out at low temperature, which requires a refrigerating plant and a corresponding consumption of refrigerating energy.
While methanol scrubbing succeeds in removing all catalyst-deteriorating components and acid gases H.sub.2 S and COS, the process in spite of the high expenditure involved, enables a selective desulfurization only to such an extent that the scrubbed gases are only slightly enriched with hydrogen sulfide and for this reason the latter cannot be processed in some cases by the Claus process to produce elementary sulfur unless an additional treatment to increase the concentration of H.sub.2 S is adopted.
So far, no process has succeeded in satisfactorily purifying coal distillation gases by means of a single scrubbing agent so as to remove catalyst-deteriorating substances and to effect a selective desulfurization at the same time.